Prior art techniques of measuring mechanical shaft angle are numerous. One such method is to use an electromechanical device generically called an resolver type position transducer. A typical resolver type position transducer would be comprised of two disks, one which rotates relative to the other. One of the disks would house the stator windings (does not move) while the other houses the rotor winding.
Generally, two separate windings are physically arranged on the stator so that electrical signals from these two windings are coupled in quadrature onto the rotor windings of the second member. The magnitude of the coupling for each of these winding pairs is dependent upon the mechanical shaft angle present between the stator and the rotor. Very often, numerous winding poles are configured into the device so that an electrical/mechanical geardown ratio is achieved. For example, a mechanical rotation of 180 degrees might correspond to an electrical phase shift of 18,000 degrees if the geardown ratio were 100 (corresponding to 200 pole pairs).
The relation between the input and output signals of an resolver type position transducer are related to the geometry of the resolver type position transducer stator and rotor windings. Since this geometry changes as the rotor changes its position, then it becomes possible to extract positional information by comparing the output and input signals of resolver type position transducer. Various methods of accomplishing this measurement have been devised using both amplitude and phase modulation techniques. U.S. Pat. No. 4,339,700 entitled "High Frequency Control System Using Digital Techniques" issued July 13, 1982 provides additional explanation of resolver type position transducer.
The present invention relies upon the fact that an resolver type position transducer can be used as a phase shifter of an input carrier frequency. Although the basic resolver type position transducer concept represents a technique which is well known to those versed in the state-of-the-art, the present invention is an improvement over the prior techniques resulting in greater accuracy of measurement. The invention also provides improvement in the linearity of measurement as opposed to alternate techniques such as those using multiplying D/A converters (an amplitude modulation technique). U.S. Pat. No. 4,339,700 also discloses the use of multiplying D/A converters.
The present invention is intended for extremely high-precision servo feedback systems which must adjust a rotating shaft to an accuracy of better than 1 part in 5,000,000 parts of a total rotation. This accuracy is achieved by a combination of two effects. The first effect is the result of decomposing a full 360 degree rotation mechanically into a much larger number of electrical rotations of phase. This aspect is achieved in the construction of the resolver type position transducer as described previously with numerous pole parts present in the inductosyn construction. Thus, in order to achieve an accuracy of 1 part in 5,000,000 for an resolver type position transducer which had 100 pole pairs, it would be necessary to design an electrical circuit capable of measuring an electrical phase angle to within 1 part in 50,000. The purity and accuracy of the signals to excite the resolver type position transducer must also have the same high precision as the circuits used to measure differences in input and output signal characteristics.
FIG. 1 represents a prior art technique. In this arrangement, quadrature carrier frequencies having the same amplitude are provided by an oscillator 101 and are fed into the two resolver type position transducer stator inputs 121, 122. The output signal from the rotor will be a carrier having the same frequency but with a phase shift added to it. If one were to compare this phase shifted signal with another version of the reference phase shifted by accurate methods, then the difference between these two signals would result in a null output from phase comparator 108 for a specific mechanical shaft angle. The error signal could then be used to direct a servo motor to adjust shaft angle in order to reduce the error.
The problems encountered in the prior art are several and have, in fact, resulted in a tendency to avoid the use of phase shifting methods. The major problems encountered in the prior art phase shifting approaches are: